In aluminum heat exchangers used for air conditioners in automobiles, in general, a plurality of fins are disposed at a narrow interval and tubes for supplying a refrigerant are disposed at the fins in a complicated manner in order to maximize a surface area from the viewpoint of improvement in heat exchange rate. In the heat exchangers having the above-described complicated structure, moisture in the air adheres to surfaces of the fins and tubes (hereinafter referred to as “fins and so forth”) as condensed water during operation of the air conditioner. In the case where wettability of the surfaces of the fins and so forth is unsatisfactory, the adhered condensed water becomes semi-spherical water droplets or exists between the fins in the form of a bridge to undesirably increase ventilation resistance, thereby raising a problem of a reduction in heat exchange rate due to the inhibition of smooth flow of exhaustion. Therefore, for the purpose of imparting hydrophilicity to surfaces of the fins and so forth, a hydrophilization treatment is ordinarily performed.
Also, aluminum and an alloy thereof forming the fins and so forth are the materials which are essentially excellent in antirust property. However, when the condensed water stagnates on the surfaces of the fins and so forth for a long time, oxygen concentration cells are locally formed to cause a corrosion reaction to proceed. Further, if contaminant components in the air adhere to the cells to be concentrated, the corrosion reaction is promoted. A product generated by the corrosion reaction such as white rust is problematic since it is deposited on the surfaces of the fins and so forth to inhibit heat exchange characteristics and is discharged in the air by a blower.
Accordingly, various techniques for improving corrosion resistance through suppression of generation of white rust have been proposed. For example, as a chemical conversion treatment agent capable of imparting favorable corrosion resistance to a surface of aluminum or an alloy material thereof, a chemical conversion treatment agent including a titanium complex fluoride ion, a pentavalent vanadium compound ion, and a zirconium complex fluoride ion has been disclosed (see Patent Document 1).
Also, as a chemical conversion treatment agent capable of imparting favorable corrosion resistance to a surface of an aluminum heat exchanger, a chemical conversion treatment agent including a decavanadate ion equivalent to the pentavalent vanadium compound ion and a zirconium complex fluoride ion has been disclosed (see Patent Document 2).
By the way, the aluminum heat exchanger used for automotive air conditioners is produced by disposing and assembling the fins and so forth as described above, followed by bonding. Since the strong and dense oxide film is formed on the surface of aluminum, it has not been easy to attain the bonding by the brazing method which is not a mechanical bonding method, and devises such as brazing in vacuum have been needed in the bonding.
As the countermeasure, flux brazing methods using a halogen-based flux have recently been developed as a means for effectively removing the oxide film on the surface, and, among them the Nocolok brazing process (hereinafter referred to as “NB process”) including brazing in a nitrogen gas has widely been employed from the viewpoints of easy brazing control and a low processing cost. With the NB process, the fins and so forth are disposed and assembled, and then the fins and so forth are subjected to the brazing using the flux such as KAlF4 and K2AlF5 in a nitrogen gas.
However, in the aluminum heat exchanger produced by the NB process (hereinafter referred to as “NB heat exchanger”), the flux inevitably remains on the surfaces of the fins and so forth. Therefore, a surface state (potential state and the like) of the fins and so forth is fluctuated to make it difficult to obtain a uniform chemical conversion film and a uniform hydrophilic film by a post-treatment. Thus, a problem of failing to obtain favorable corrosion resistance and hydrophilicity has been raised.
Accordingly, as an NB heat exchanger surface treatment method capable of imparting the favorable corrosion resistance and hydrophilicity as well as a deodorizing property which is one of important characteristics for use as an automotive air conditioner, a hydrophilization technique including performing a chemical conversion treatment by dipping the NB heat exchanger into a chemical conversion treatment agent including at least one of a zirconium complex fluoride ion and a titanium complex fluoride ion and then dipping the NB heat exchanger into a hydrophilization treatment agent including polyvinyl alcohol, polyoxyalkylene-modified polyvinyl alcohol, an inorganic crosslinking agent, a guanidine compound, and so forth has been disclosed (see Patent Document 3).
Also, as a surface treatment method capable of maintaining hydrophilicity, high corrosion resistance, and an antibacterial and deodorizing property on the surface of an aluminum or aluminum alloy base material for a long time, a technique including a surface adjustment step for adjusting a surface of an aluminum or aluminum alloy base material to the one which is suitable for formation of a chemical conversion film, a step for washing with water, a step for forming on the surface of the aluminum or aluminum alloy base material a first protection layer including the chemical conversion film, a step for washing with water, a step for coating the first protection layer with a second protection layer which is an organic film, and a drying step, the steps being performed in this order, has been disclosed (see Patent Document 4). With the technique, the first protection layer is formed by using a chemical conversion treatment liquid including vanadium and at least one kind of metal selected from among titanium, zirconium, and hafnium, and the second protection layer is formed by using a composition including (1) a chitosan derivative and a solubilizing agent, (2) modified polyvinyl alcohol obtainable by graft polymerization of a hydrophilic polymer on a side chain of polyvinyl alcohol, and (3) a water-soluble crosslinking agent.
[Patent Document 1] Japanese Unexamined Patent Application, Publication No. 2010-261058
[Patent Document 2] Japanese Unexamined Patent Application (Translation of PCT Application), Publication No. 2004-510882
[Patent Document 3] Japanese Unexamined Patent Application, Publication No. 2006-69197
[Patent Document 4] Japanese Unexamined Patent Application, Publication No. 2011-161876